SUMMARY microRNAs, non-ion channel proteins and the control of drug-induced arrhythmia Heart failure (HF) is a major cause of morbidity and mortality worldwide and affects 1?2% of the population; thus, it represents a major public health burden. HF patients are particularly sensitive to arrhythmia. Indeed, as many as 50% die from arrhythmias and sudden cardiac death, creating a precarious health risk, yet few drugs are effective in this population. This R01 application addresses two major questions relevant to the clinical management of these patients: 1) Why are HF patients particularly susceptible to drug-induced arrhythmia (Aim 1) and 2) Can we discover proteins that can be targeted pharmaceutically to create entirely new classes of drugs to treat arrhythmia in these patients (Aims 2 and 3)? Most current molecular-level knowledge of arrhythmia proclivity involves ion channels, and most anti- arrhythmics target ion channel proteins. Ion channel function, however, is highly regulated at multiple levels including translation, trafficking, turnover, and gating. We propose that elucidating such mechanisms at the molecular and systems levels will profoundly deepen our understanding of the variable responsiveness to drugs and point to new therapeutic targets. We will use iPSC-derived cardiomyocytes (iPSC-CMs) carrying dilated cardiomyopathy (DCM)- causing genes to recapitulate the proarrhythmic and contractile dysfunction of HF in vitro. DCM and isogenic control iPSC-CMs will be used in high throughput screening studies to explore the basis for drug proarrhythmia and identify novel therapeutic targets. To date, physiological screening of cardiomyocytes has lacked the throughput and prediction accuracy needed to systematically probe mechanisms and discover therapeutic targets. We will leverage over a decade of research and development of instrumentation and iPSC-CM technology to overcome this limitation. The conclusions drawn from iPSC-cardiomyocytes about mechanisms and candidate drug targets will be evaluated in lower throughput adult cardiomyocyte preparations and in multiscale computational modeling (with subcontract) to confirm and extend hypotheses, thereby generating new insight relevant to the intact human heart. Our team is highly synergistic and uniquely appropriate to carry out this large-scale project to address the challenges of managing arrhythmia in heart failure patients, and ultimately to create more effective therapeutics.